The invention relates to implantable cardiac pacers and more particularly to a rate responsive pacer which automatically alters the stimulation rate of the heart in response to the metabolic demand of the patient. At the present stage, a wide range of cardiac pacers is used to replace, by an implantable device, the mechanisms producing and driving the electrical signals associated with the heart function in the human body. These pacers are identified by internationally agreed abbreviations. Hereafter we will use the word "Pacemaker" (registered trademark) in place of implantable cardiac electrostimulator. The pacers wherein the stimulation of the ventricle is driven and synchronized with the naturally occurring activity of the atrium are generally considered physiologic as the artificial stimulation rate is not preset but is induced by the atrial natural activity and is, therefore, always proportional to the latter one. As a consequence this kind of pacemaker can be used only in those cases wherein the atrial activity is physiologic, that is, only 30/60% of the present pacemaker patients. Hence, in the present conditions a high percentage of patients cannot use an electrostimulation based on a physiologically variable rate according to the above description. This is the reason why many experts have long tried to realize an implantable pacemaker able to detect an alternative parameter other than the atrial activity, parameter which can change in response to the physiologic demand of the patient and can be used as a reference variable in order to automatically and physiologically regulate the heart electrostimulation rate. As yet the pacemakers theorized and designed to solve said problem are those hereafter summarized wherein the electrostimulation rate is subject by an algorithm to changes in the following parameters: atrial activity--blood pH--body temperature--oxygen saturation in venous blood--QT interval obtained from the endocavitary E.C.G.--respiratory rate--mechanical activity--cardiac output--electromyogram of the diaphragm. An overall discussion on the characteristics of these proposals can be found in:
Clin. Prog. Pacing and Electrophysiol. Vol. 1 n. 1--1983 "Rate Responsive Pacing" by Anthony F. Rickards M.D. and Robert M. Donaldson M.D.--From the National Heart Hospital--London W 1 England -
and in:
"A Physiologically Controlled Cardiac Pacemaker"--Krasner--Voukydis and Nardella--J.A.A.M.I., Vol. 1 n. 3--1966; 14-20--
and more specifically in:
"A Pacemaker which automatically increases its rate with physical activity" by Kenneth Anderson, Dennis Brumwell, Steve Huntley--From Medtronic Inc.--Minneapolis--Minn.--U.S.A.
and:
"Variation of Cardiac Pacemaker rate relative to respiration"--IEEE/Engineering in Medicine and Biology Society First Annual Conference--p. 50, 1979--by Getzel W., Orlowski J., Berner B., Cunnigham B., Esser M., Jacob M., Jenter D. Other pacemakers of the kind are described in the European patent application No. EP-A-0 080 348 as well as in U.S. Pat. Nos. 3,593,718 --4,228,803--4,313,442--4,202,339--4,140,132--4,009,721--and in the European patent application No. EP-A-0 089 014 and corresponding U.S. application Ser. No. 06/474,241 filed by the same applicants of the present application.
All the literature highlights the existing need and desire to realize a pacemaker able to adjust the stimulation rate to the metabolic demand by means of a suitable physiologic variable sensor circuit. The present invention consists of a rate responsive pacer the inhibition and/or stimulation rate of the ventricle and/or of the atrium of which is automatically driven in the range between minimum and maximum programmable values by a measuring and processing circuit which detects the variations in time of the geometry of a part of the chest of the patient as a consequence of pulmonary ventilation. The circuit also produces a signal which depends on said physiologic variable and which automatically changes the pacemaker stimulation rate between said minimum and maximum values.